A pebble-bed high-temperature gas-cooled reactor originated from an AVR experimental reactor in Germany Based on success of the AVR reactor experiment, in the 1970s, i.e. in the high-speed development period of world nuclear power, the thorium high-temperature gas-cooled reactor demonstration power plant (THTR-300) with electric power of 300 MW was built and operated in Germany. With occurrence of the nuclear power plant accidents of Three Mile Island in the USA and Chernobyl in the Soviet Union, the public and supervision authorities in various countries increasingly pay more attention to safety of nuclear power plants, and thus, the development trend of high-temperature gas-cooled reactor commercial power plant is changed into a modular high-temperature gas-cooled reactor with passive inherent safety from original large-scale direction. The high-temperature gas-cooled reactor nuclear power plant demonstration project (HTR-PM) of the HuaNeng ShanDong ShiDao Bay nuclear power plant, which has been constructed in China, is a typical modular pebble-bed high-temperature gas-cooled reactor.
Due to high single-reactor power of the thorium high-temperature gas-cooled reactor (THTR-300), two sets of control rod systems are arranged, wherein one set of control rod system is provided with 36 regulating rods that are arranged on a lateral reflection layer and used for regulating rapid reactivity change and hot shutdown under the accident working condition; and the other set of control rod system is provided with 42 control rods and the 42 control rods are inserted into a reactor core pebble bed and are used for carrying out long-term cold shutdown and ensuring a certain cold shutdown depth. The operating experience of the thorium high-temperature gas-cooled reactor shows that the control rods inserted into the reactor core pebble bed require a huge driving force to overcome resistance of stacked spherical fuel elements, which causes damage to the fuel elements. Therefore, only the lateral reflection layer control rods are retained in the later design of the modular pebble-bed high-temperature gas-cooled reactor HTR-MODUL.
The HTR-MODUL has single-reactor thermal power of 200 MW, a reactor core diameter of 3 m and a reactor core average height of 9.4 m. Reactivity control and Shutdown systems of the HTR-MODUL include a control rod system and an absorption sphere shutdown system, the control rod system is provided with six control rods in total, the six control rods are arranged at the lateral reflection layer, and each control rod corresponds to a set of driving mechanism for enabling the control rod to move up and down. Each control rod has an absorber length of 4,800 mm and a total length of 5,280 mm and is divided into ten sections in total, each control rod has an outer diameter of 10.5 mm, each control rod has a pore diameter of 130 mm, a cladding material adopted by the control rods is X8CrNiMoNb 1616, the total weight of each control rod is 104 kg, and the highest design temperature of each control rod is about 900 DEG C. The control rod system of the HTR-MODUL has the main functions of reactor power regulation and hot shutdown.
Design parameters of the control rods show that the control rods are of a multi-section single-rod structure; since all the control rods need to be taken out of an active region when the reactor operates under the full power, the length of an absorber of each control rod is about half the height of the reactor core active region due to the limitation of the height of a reactor pressure vessel.
Except for the control rod system, the other set of reactivity control and shutdown system of the HTR-MODUL is the absorption sphere shutdown system. The system is provided with 18 lines of absorption spheres which are also positioned at the lateral reflection layer of the reactor, the absorption spheres fall into pore passages of the lateral reflection layer by means of gravity and are returned to a sphere storage tank from the pore passages of the lateral reflection layer in a pneumatic conveying manner. The system has the main functions that: 1, when the reactor is started and operates under low power, the absorption sphere shutdown system works together with the control rod system to carry out reactivity control; and 2, the absorption sphere shutdown system separately achieves cold shutdown and ensures a certain cold shutdown depth.
The above mentioned reactivity control of the HTR-MODUL has the following problems that: 1, the absorption sphere shutdown system has a great number of functional requirements, is complex in system design and has a high requirement for operation reliability; 2, when the reactor is started and operates under low power, a reactor operator not only needs to operate the control rod system, but also needs to blow the absorption spheres up in a pneumatic conveying manner from each absorption sphere pore passage, and the amount of the absorption spheres conveyed each time needs to be accurate and controllable, which brings great operation difficulty to the reactor operator and is likely to cause accidents.
Therefore, design of the HTR-PM reactivity control and shutdown system adopts the following technical innovations that: two sets of mutually independent systems, the control rod system and the absorption sphere system, are still retained but the functions of the two sets of systems are regulated; the control rod system is divided into safety rod banks, regulating rod banks and shim rod banks, the safety rod banks are all taken out of the reactor active region when the reactor is started and operates under low power, the value of the safety rod banks is sufficient to ensure shutdown under any reactor working condition, the regulating rod banks perform reactor power regulation so as to flatten reactor core power distribution and compensate reactivity change of a reactor core during the normal operation, the shim rod banks are used for compensating reactivity change after the reactor operates for a long time; as actuating mechanisms of a reactor protection system, the safety rod banks and the regulating rod banks can rapidly achieve hot shutdown, assumed that one control rod with the highest reactivity value is in failure; and when the reactor is started and operates under low power, the regulating rod banks and the shim rod banks cooperate to carry out reactivity control. If all the control rods are put in. Cold shutdown can be separately achieved and a certain cold shutdown depth is ensured. As a standby shutdown system, the absorption sphere shutdown system does not participate in startup of the reactor and operation at all power levels and can be manually put into use as required; and when the control rod system and the absorption sphere shutdown system are put into use together, long-term cold shutdown or overhaul shutdown can be achieved.